Biomedical subjects

A M Kappers

Publications and source records attributed to A M Kappers.

At least 37 records · Page 2Linked to original sources

PubMed1997

Perceptual localization of surface position.

In 2 experiments, observers were required to identify corresponding points on an object viewed from multiple orientations. On each trial, a surface was presented initially with a single target location marked by a small dot. Following a brief blank interval, the same surface was presented again at a different orientation. The observer was required to position an adjustable probe dot in this 2nd display to match the location of the target in the 1st view. Under optimal conditions, the variance in their settings over multiple trials was just a few minutes of arc, though these errors varied significantly with the structural complexity of the depicted surface.

Adult↗

PubMed1997

Investigation into the origin of the haptic aftereffect of curved surfaces.

In haptics, the perceived (phenomenal) flatness of a surface is strongly influenced by a previous surface which has been statically touched. The mechanisms underlying this haptic aftereffect of curved surfaces are investigated. It is shown that the representation of curvature abstracted from the sense of touch, ie a high-level representation, is not affected during the aftereffect. This is concluded because: (1) the aftereffect does not exhibit intermanual transfer; (2) the way in which two successive surfaces are touched can influence the magnitude of the aftereffect; and (3) it is not necessary to touch a surface-active muscular contraction can also result in a shift of the phenomenal flatness. Furthermore, it is suggested that the physiological process involved in the aftereffect is a central process, ie it is located in the brain but it is distinct for each hemisphere. This is supported by the findings that: (1) the decay rate of the aftereffect is not influenced by the degree of peripheral stimulation during the decay; and (2) the aftereffect does not transfer from the adapted hand to the unadapted hand.

Humans↗

PubMed1997

Large scale differences between haptic and visual judgments of curvature.

Large systematic scale differences show up when subjects are asked to match the curvature of a haptically presented surface with a visually presented cross section. Scaling factors ranging from 0.3 to 3.0 were found. Most often (for five out of six subjects) haptically estimated curvature was overestimated relative to visual curvature. We conclude that mutually inconsistent representations of surface curvature coexist in a single observer in the haptic and visual modalities. Another conclusion can be drawn with certainty, namely that at least one of the two modalities does not provide the observer with veridical information. The results for the left and the right hand of a subject were very similar, though for two subjects there existed a small significant difference in scaling. It seems likely that the curvature representations of the two hands are not independent of one another, but our data do not permit us to be conclusive.

Adult↗

PubMed1997

Effects of texture, illumination, and surface reflectance on stereoscopic shape perception.

Observers viewed computer-generated stereograms of randomly structured smooth surfaces and were required to judge the perceived local orientation at numerous probe points by adjusting a monocular gauge figure. The surfaces were depicted with specular or Lambertian reflectance functions, either with or without identifiable texture elements, and with varying directions of illumination. The results revealed a strong linear correlation between the judged patterns of relief and the actual depicted objects, though there were systematic differences in the magnitude of depth scaling in the different conditions. In general, the accuracy and reliability of observers' judgments for the smoothly shaded shiny surfaces was slightly lower than for the textured surfaces and slightly higher than for the smoothly shaded Lambertian surfaces. The direction of illumination had no detectable effect on the observers' judgments.

Computer Graphics↗

PubMed1997

Detection of second-order structure in optical flow fields.

We investigate how well the direction of the expansion gradient and the direction of a spatial step in expansion can be extracted from a complex optical flow field. The expansion gradient is an invariant of the second-order structure under rotations of the image plane. Our stimuli consist of sparse random dot patterns in a circular window with a diameter of 20 degrees. We find that translational velocity masks the direction of both the expansion gradient and the step in expansion. Thresholds for both the expansion gradient and the step in expansion are found to be similar for a wide range of expansions. We introduce two simple models that use the thresholds for the step in expansion to predict thresholds for the expansion gradient. Results of these models are comparable with the results of our subjects, showing that we do not need a hypothetical separate mechanism for the detection of second-order structure to explain the data.

Female↗

PubMed1997

Correspondence in pictorial space.

We have investigated psychophysically determined image correspondences between pairs of photographs of a single three-dimensional (3-D) object in various poses. These correspondences were obtained by presenting the pictures simultaneously, side by side, and letting the subject match a marker in one picture with a marker (under manual control) in the other picture. Between poses, the object was rotated about a fixed vertical axis; thus, the shifts of the veridical correspondences (with respect to the surface of the object) were very nearly horizontal. In fact, the subjects produced appreciable scatter in both horizontal and vertical directions. The scatter in repeated sessions and between data depends on the local (landmarks) and global (interpolation) structure of the pictures. Since the object was fairly smooth (white semigloss finish) and nontextured, the only way to establish the correspondence is by way of the "pictorial relief." The relief is some largely unknown function of the image structure and the observer. Apparently, more immediate entities (e.g., the shading or the contour) cannot be used as such, since they vary with the pose. We compare these data with results obtained with a surface attitude probe on a single picture. We studied various measures of consistency both within a single method and between methods. We found that subjects were confident in establishing correspondences, but results scattered appreciably in a way that depended on both global and local image structure. Correspondence results for various pose angles were mutually very consistent, but only to a minor extent with results of attitude measurements. The main finding was that subjects could establish correspondence on the basis of their 3-D interpretation (pictorial relief), even if the 2-D graytone distributions are quite different.

Form Perception↗

PubMed1997

The visual contour in depth.

We asked subjects to match points on the surface of a smooth three-dimensional (3-D) shape with points on the surface of another object that was geometrically identical to the first object but was placed in a different pose, was differently textured, and was differently shaded. In all cases, the fiducial point was on the rim of one of the objects (i.e., the boundary of the visible region of the surface), whereas the matching point was well within the silhouette of the other object. This allowed us to draw (preliminary) conclusions concerning the way monocular human observers are able to handle the neighborhood of the rim, where the local slant assumes arbitrarily high values. All experiments were done in real space with real objects (no computer-simulated scenes), the points being indicated with laser beam illumination. The subject was given control over the direction of the laser beams and was thus able to perform the task by adjustment from the vantage position. We studied both consistency (whether the subject's judgments were invariant against changes of relative pose) and veridicality (whether the depth of the visual contour as calculated from the settings agreed with the true distance as measured by mechanical means). Subjects caught much of the 3-D structure of the contour but did deviate appreciably and apparently idiosyncratically from the true geometry.

Computer Graphics↗

PubMed1997

Detection of spatial discontinuities in first-order optical flow fields.

We investigated the extent to which the human visual system can detect discontinuities in first-order optical flow fields. We constructed two types of spatial discontinuities: a circular split field with a straight edge and a disk with annular surround. We used two different first-order optical flow components: an expansion and a rotation. We found an intriguing difference in the detection thresholds for straight and circular discontinuities. Whereas straight discontinuities yielded thresholds of 10%-50% difference in expansion or rotation, circular discontinuities could, at first, only be detected at extreme differences (>> 100%). After a learning period, thresholds for such stimuli decreased, but they remained significantly higher than thresholds for the straight edge. Thresholds rose for stimuli that formed a gradual transition between a circular and a straight edge, and they decreased with increasing eccentricity of the circular discontinuity. Results suggest that symmetry in this stimulus, defined by the coincidence of the center of expansion or rotation and the center of the circular discontinuity, was responsible for the difference in thresholds for circular and straight discontinuities.

Adult↗

PubMed1997

Haptic curvature discrimination at several regions of the hand.

Static haptic discrimination of the curvature of convex, concave, or straight 20-cm-long strips was investigated for nine placements on the hand. In one condition, the strips were touched with the palmar side of the hand, and in the other condition, with the dorsal side. The influence of the lengths of the strips, and thus of contact lengths, was also investigated. For all placements, discrimination was poorer in the dorsal than in the palmar condition, owing to poorer cutaneous resolution on the dorsal side of the hand (the kinesthetic stimulation was the same in both conditions). Thus cutaneous stimulation is important. In both experiments, performance appeared to depend primarily on contact length. Moreover, the discrimination thresholds for all different placements and contact lengths followed the same trend. We conclude that in these experiments the effective stimulus for the discrimination of curved strips is the total difference of local surface attitude--that is, the slope difference over the far ends of the stimulus.

Adult↗

PubMed1997

Monocular discrimination of rigidly and nonrigidly moving objects.

We measured thresholds for the monocular discrimination of rigidly and nonrigidly moving objects defined by motion parallax. The retinal projections of rigidly moving objects are subject to certain constraints. By applying smooth 2-D transformations to the projections of rigidly moving objects, we created stimuli in which these constraints were affected. Thresholds for (generic) nonrigid transformations that in theory can be detected from rigid ones by processing pairs of views depended not only on the extent to which the rigidity constraints were affected, but also on the structure and the movement of the simulated object. Nonrigid transformations under which every three successive views had a rigid interpretation were not discriminable from rigid transformations, except in cases where the distortions were very large. Under the rigidity assumption, this would mean that a large class of nonrigidly moving objects is erroneously perceived as rigidly moving.

Adult↗

PubMed1996

Detection of first-order structure in optic flow fields.

We measured psychophysical thresholds for the detection of four different optic flow components in the presence of a translational velocity. We also measured thresholds for detection of rotation in the presence of expansion and for expansion in the presence of rotation. Our stimuli consisted of sparse random dot patterns. Detection thresholds are similar for all four optic flow components. Thus, our experiments indicate that our subjects use a factor that is similar in all first-order flow components, namely the relative orientation of the velocity vectors. First-order components can be extracted independently of both each other and translational velocity.

Humans↗

PubMed1996

Simulating the detection of first-order optical flow components.

Thresholds for the detection of rotation and divergence in the presence of a translational component in sparse random dot patterns are determined for human observers and two computer algorithms. The algorithms only make use of local velocity directions and not of local velocity magnitude (speed). The results show that psychophysical performance in this task can be well described without the need of specialized mechanisms tuned to either rotation or divergence. Possibly, integration of information over more than two frames occurs for low velocities. For high velocities the correspondence problem seems to limit performance.

Algorithms↗

PubMed1996

Effects of changing viewing conditions on the perceived structure of smoothly curved surfaces.

Observers' perceptions of a male and a female torso were investigated using monocular and stereoscopic images under varying conditions of illumination. Observers judged the shapes of these torsos by adjusting a gauge figure to estimate the local slant and tilt at numerous probe points arranged in a lattice over the torso's surface. The results revealed that the judged surfaces in the monocular and stereoscopic conditions were related by an affine stretching transformation in depth that accounted for approximately 95% of the between-condition variance. There was also a strong affine component between the judgments obtained for the different illumination directions, although a further analysis of the residuals indicated that changing the direction of illumination influenced perceived structure in a piecewise manner.

Contrast Sensitivity↗

PubMed1996

Surface range and attitude probing in stereoscopically presented dynamic scenes.

The authors report on different methods to probe the structure of visually perceived surfaces in 3 dimensions. The surfaces are specified by patterns of shading with Lambertian and specular components, which deform over time and over stereoscopic views. Five observers performed 2 probe tasks, 1 involving the adjustment of a punctate probe so as to be on the apparent surface and the other involving the adjustment of a small gauge figure that indicates surface attitude. The authors found that these rather different methods yielded essentially identical depth maps up to a linear transformations and that the observers all deviate slightly from veridicality in basically identical ways. The nature of this deviation appears to be correlated with the rough topography of the specularities.

Attention↗

PubMed1996

Haptic aftereffect of curved surfaces.

A haptic aftereffect of curved surfaces is demonstrated. Two spherical surfaces were presented sequentially to human subjects. They rested one hand on the first (conditioning) surface. After a fixed conditioning period they transferred their hand to the second (test) surface and judged whether the test surface was convex or concave. In experiment 1 the curvature of the conditioning surface was varied; the subject's judgment of convexity or concavity of the test surface was strongly shifted in the direction opposite to the curvature of the conditioning surface (negative aftereffect). Therefore, subjects judged a flat surface to be concave after being exposed to a convex surface. After a conditioning period of 5 s the shift was about 20% of the curvature of the conditioning surface. In experiment 2 the duration of the conditioning period was varied; the magnitude of the aftereffect could be described by a first-order integrator with a time constant of 2 s. In experiment 3 the time interval between the conditioning period and the touching of the second surface was varied; the magnitude of the aftereffect could be described by an exponential decay with a time constant of 40 s. It is concluded that the haptic aftereffect of curved surfaces is an important effect that occurs almost instantaneously and lasts for an appreciable period.

Adult↗

PubMed1996

Haptic unilateral and bilateral discrimination of curved surfaces.

Active haptic discrimination of cylindrically curved hand-sized surfaces was investigated. Unilateral discrimination (that is, with one hand, either the left or the right one) was compared with bilateral discrimination. In this latter condition, the right hand had to touch the right surface, and the left hand the left one. The importance of which surface of the pair is placed to the right of the observer and which to the left (the so-called placement order) was also investigated. In comparison with existing studies, performance in the present experiments was better owing to the combined effects of a larger surface area and active instead of passive discrimination. The results show clearly that curvature discrimination does not follow a Weber law; performance is relatively better with the larger curvatures. For all subjects, unilateral discrimination was much better than bilateral discrimination. A partial cause of this difference is the influence of the placement order, which is very apparent in the results for the bilateral condition. These results cannot be fully explained but the findings suggest an influence of the object-observer relation on the perception of the object.

Discrimination, Psychological↗

PubMed1996

Detection of the sign of expansion as a function of field size and eccentricity.

Small optical flow fields are involved in object and shape recognition. These tasks depend on high resolution and low speed of the flow-field elements. Large optical flow fields are involved in tasks such as orientation and navigation, which require flow-field elements to move at high speed. From the above, we would expect to find different behavior of subjects for different parameter ranges of field size and speed of the elements. In this paper, we address the question of whether such different behavior exists for a task that is associated neither with object recognition nor with navigation. We obtained detection thresholds for expansion in the presence of translation for a wide range of field sizes. The same paradigm was used to investigate whether subjects made efficient use of peripheral information. We found viewing-distance invariance, meaning that subjects' performance scaled with stimulus size. Subjects performed very similarly with and without foveal information.

Adult↗

PubMed1996

Structure from motion: a tolerance analysis.

We present a tolerance analysis that is applicable to a large group of stimuli used in structure-from-motion tasks. Human performance in structure-from-motion tasks reflects the fact that the visual system deals with projections of a 3-D world on the retina. A tolerance analysis reveals the relationship between the projections and the 3-D world. Any realistic model of the visual system should incorporate a tolerance analysis as a complete description of the stimulus. By way of example we apply the tolerance analysis to the stimuli used in two widely known experiments in which different properties of structure were tested--that is, perceived nonrigidity (Norman & Todd, 1993) and ordering in depth (Hildreth, Grzywacz, Adelson, & Inada, 1990). The analysis explains qualitatively the results of these experiments, illustrating that the results are to a large extent due to stimulus limitations rather than to mechanistic properties of the visual system. From our analysis it follows that far more sensitive measurements of the optic information are needed to obtain metric structure than affine structure.

Depth Perception↗